<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>id选择器</title>
    <style>
        #box1{
            background-color: yellow;
        }
        #box2{
            background-color: red;
        }
        #box3{
            background-color: blue;
        }
    </style>
</head>
<body>
    <div id="box1">111111</div>
    <div id="box2">222222</div>
    <div id="box3">333333</div>
    <!-- 
        id具有唯一性 id里不能有两个身份 
        不能多个标签共享一个id 也不能一个标签有两个id
        -->
</body>
</html>